With the recent rapid progress of potable electronic equipment and communication equipment, secondary batteries having a high energy density are strongly desired from the standpoints of economy and size and weight reduction. Prior art known attempts for increasing the capacity of such secondary batteries include the use as the negative electrode material of oxides of V, Si, B, Zr, Sn or the like or compound oxides thereof (JP-A 5-174818, JP-A 6-60867 corresponding to U.S. Pat. No. 5,478,671), melt quenched metal oxides (JP-A 10-294112), silicon oxide (Japanese Patent No. 2,997,741 corresponding to U.S. Pat. No. 5,395,711), and Si2N2O or Ge2N2O (JP-A 11-102705 corresponding to U.S. Pat. No. 6,066,414). Also, for the purpose of imparting conductivity to the negative electrode material, mechanical alloying of SiO with graphite followed by carbonization (JP-A 2000-243396 corresponding to U.S. Pat. No. 6,638,662), coating of Si particle surfaces with a carbon layer by chemical vapor deposition (JP-A 2000-215887 corresponding to U.S. Pat. No. 6,383,686), coating of silicon oxide particle surfaces with a carbon layer by chemical vapor deposition (JP-A 2002-42806), and coating of silicon or silicon alloy particles with a polyimide binder followed by sintering (JP-A 2004-22433 corresponding to U.S. 2003-0235762A) are known.
These prior art methods are successful in increasing the charge/discharge capacity and the energy density of secondary batteries, but fall short of the market demand and are not satisfactory because of insufficient cycle performance, a substantial volume change of the negative electrode film itself upon charging/discharging cycles, and separation from the current collector.
Some of these problems can be solved by carbon surface coating by thermal CVD. The charge/discharge capacity is more than the necessity at the present when a balance thereof with the low capacity of current positive electrodes is taken into account. In some cell applications, there is a need for a material having better cycle performance even at some expense of capacity. It would be desirable to have a negative electrode active material which is based on silicon and exhibits better cycle performance despite some reduction in energy density.
More particularly, Japanese Patent No. 2,997,741 describes a high capacity electrode using silicon oxide as the negative electrode material in a lithium ion secondary cell. As long as the present inventors have empirically confirmed, the performance of this cell is yet unsatisfactory due to an increased irreversible capacity on the first charge/discharge cycle and a practically unacceptable level of cycle performance. With respect to the technique of imparting conductivity to the negative electrode material, JP-A 2000-243396 provides insufficient conductivity since a uniform carbon coating is not formed due to solid-solid fusion. JP-A 2000-215887 is successful in forming a uniform carbon coating, but the negative electrode material based on silicon experiences extraordinary expansion and contraction upon absorption and desorption of lithium ions and as a result, fails to withstand practical service. At the same time, the cycle performance declines, and the charge/discharge quantity must be limited in order to prevent such decline. In JP-A 2002-42806, an improvement in cycle performance is ascertainable, but the capacity gradually decreases with the repetition of charge/discharge cycles and suddenly drops after a certain number of cycles, because of precipitation of silicon micro-crystals, the under-developed structure of the carbon coating and insufficient fusion of the carbon coating to the substrate. This negative electrode material is yet insufficient for use in secondary batteries.